Within digital and optical communications systems, a carrier frequency offset may refer to a difference between the carrier frequency at a transmitter and the carrier frequency at the receiver. For instance, the transmitter may transmit at the nominal carrier frequency. At the receiver, an unmodulated frequency may be required for reception of the transmission, however it may not be physically possible to have the carrier frequency at the receiver exactly match the carrier frequency at the transmitter. Thus, this offset between frequencies may be described as the carrier frequency offset. Causes for this offset may include temperature change, mechanical vibration and etc. Accordingly, reduction of the carrier frequency offset through frequency and phase tracking (e.g., frequency recovery) may greatly improve the overall performance of the digital communications system.
Typically, a carrier recovery system may be used to estimate and compensate for frequency and phase differences between a carrier wave of a received signal and a local oscillator of the receiver for the purpose of coherent demodulation. While carrier recovery may be accomplished with an optical phase-locked loop (“PLL”), these methods are very complex. Conventional digital PLL-based blind carrier recovery algorithms have the capability to recover carrier phase and frequency simultaneously, and thus, is widely used for wireless systems. However, these types of algorithms cannot be used for high-speed optical system.
Unlike the wireless system in which the frequency and phase offset changes are relatively similar and slow, the characteristics of frequency and phase offsets in the optical system are very different. For example, frequency change is relatively slow (e.g., typically in the milliseconds for high-quality lasers) but the range may be large (e.g., more than 100 MHz), while the carrier phase varies much faster as compared to the wireless systems (e.g., within the nanosecond). Such characteristics will make PLL-based algorithms perform poor due to the intrinsic feedback delay. Furthermore, optical systems typically require heavily parallel processing that may further degrade the performance of these PLL-based algorithms.